The present invention relates to a method of causing sedimentation of suspended or otherwise transported sedimentary solid material at the upper surface of a porous substratum of a natural body of water, such as a lake, a sea or an ocean.
Some coastal stretches and banks along oceans, seas, lakes and rivers, as well as areas of the substratum, floor or bottom below such bodies of water are exposed to influences from waves or currents, whereby the coastlines or banks and the substratum or floor are continuously changed. As an example, sea currents may erode the coast in certain areas and transport material therefrom to other areas of the sea, where the material settles. Along certain stretches of a coast the erosion may be so vigorous that the coastline may move substantially inwards in a few years so that buildings, such as hotels, summer houses, and other installations, which were originally located at a substantial distance from the coast, are endangered. Therefore, several efforts have been made to develop efficient methods of coast protection. From old times it has been known to protect a coast by means of groynes which extend transversely to the coastline and are positioned with relatively small spacing. Such groynes are relatively efficient, but they are expensive to build and to maintain. Groynes are often used in combination with solid structures or dikes extending substantially parallel to the coast--so-called sea walls--which also resist coastal erosion. In later years experiments have been made with artificial seaweed, which is made from plastic and has been anchored to the sea floor in areas exposed to erosion. It was believed that the artificial seaweed would promote sedimentation of sedimentary material. Coast protection by means of artificial seaweed has, however, proved less efficient than expected.
An often used method of coast protection is the so-called artificial nourishment. By this method sand may be transferred from deeper areas of the sea to the beach or to the sea floor adjacent to and in front of the stretch of the coast exposed to erosion, for example by means of a suction dredger, or sand may be transported to and unloaded on the foreshore of the exposed coast by means of trucks or other vehicles. As the sand supplied in this manner is eventually removed due to the littoral drift, the artificial nourishment must be repeated at certain time intervals in order to be effective. As the known method of artificial nourishment involves handling and transport of large amounts of sand and must be repeated rather frequently, this method of coast protection is relatively expensive.
German patent specification No. 835.873 discloses a method of land reclamation in coastal areas near salt water seas. This known method involves the supply of fresh water to the salt-water saturated foreshore. As fresh water is normally not available in large quantities in coastal areas, this known method is not suited for use in a larger scale.
Proc. Civ. Engrg. in the Oceans/III, Newark, Del., 1975, Vol. I, pp. 142-160 describes an experimental study which was carried out in a water wave channel which was 50 feet long, 2 feet wide, and 3 feet deep. A perforated pipe extending along the length of the water wave channel (i.e. at right angles to the pretended 2 feet long coastline) constituted a sub-sand filter system from which water was pumped. The effectiveness of the system on stabilizing the offshore profile, on stabilizing the foreshore profile, and on decreasing the breaker scour was determined. It was found that the sub-sand filter system had a stabilizing effect on the bed material in the offshore zone, a negligible effect on breaker scour in the breaker zone, and was efficient in speeding accretion in the foreshore zone.